Specific Aims: Bronchiolitis is the leading cause of hospitalization for infants, but there is a lack of consensus about optimal management. The proposed study has 3 specific aims to address this knowledge gap: 1) To elucidate the role of multiple pathogen infections and to determine the utility of PCR-testing for an infectious etiology. 2) To create high-risk and low-risk clinical rules to predict reliably those children with bronchiolitis who would require continuous positive airway pressure or intubation and those who would not require intensive care. 3) To develop hospital discharge guidelines that encourage earlier, but safe discharges and foster family-centered care. Design: We will conduct a prospective multicenter cohort study. Over a 3-year period, researchers at 15 hospitals will enroll and have 1-week follow-up for 2,250 children <2 years hospitalized with bronchiolitis. The study will use the Emergency Medicine Network (EMNet, www.emnet-usa.org), a clinical research network that has completed >20 multicenter studies focusing on respiratory emergencies and public health. Methods: We will collect microbiologic data, demographic, birth, nutritional, family, and environmental information as well as clinical data from the emergency department, medical ward, and intensive care unit. Analysis: When comparing infants with bronchiolitis due to multiple pathogens to those with single or no identifiable pathogen, we will have 80 percent power to detect a 1.4 fold difference in CPAP or intubation and a 1.2 fold difference in mean hospital length-of-stay. Approximately 225 infants will require CPAP or intubation (high-risk outcome), while 1800 will not require intensive care (low-risk outcome). For the less common high-risk outcome, we will use 1/2 of our data to create a clinical prediction rule with up to 11 variables and test the model on the other 1/2 of our data. The tested discharge guidelines will be of sufficient precision based on calculated 95 percent confidence intervals to be useful when making clinical decisions. Relevance: By conducting research how to better treat and manage of one of the most common pediatric illnesses, our aims match well with the Healthy People 2010 focus areas of respiratory disease and maternal, infant, and child health. For children with bronchiolitis, this study will help determine the utility of testing for an infectious etiology, advance the development of evidenced-based management and triage guidelines, and encourage safe early discharges and more family-centered care.